Computed-tomography (CT) imaging is a widely-used form of medical imaging which uses X-rays to obtain three-dimensional image data. A CT image data set obtained from a CT scan may comprise a three-dimensional array of voxels, each having an associated intensity which is representative of the attenuation of X-ray radiation by a respective, corresponding measurement volume. The attenuation of X-ray radiation by the measurement volume may be expressed as an intensity value or CT value in Hounsfield units (HU), where 0 HU is the CT value of water.
In CT scanners, an X-ray source, which may be called an X-ray tube, is rotated around a patient. The X-ray radiation that passes through the patient is captured by an X-ray detector on the opposite side of the patient. The X-ray tube has a given peak tube voltage. X-ray photons are produced by the X-ray tube, the photons having a range of energies up to an energy corresponding to the peak tube voltage. For example, an X-ray tube at a peak tube voltage of 100 kV may produce photons with a range of energies up to 100 keV. A CT scan with a peak tube voltage of 100 kV may be described as a 100 kVp scan, where kVp stands for kilovolt peak.
Conventional CT acquisition, which may be called single-energy CT, may be performed with a peak tube voltage of, for example, 120 kV and a detector that is sensitive to the spread of X-ray energies provided by the X-ray tube.
A limitation of single-energy CT imaging may be that different materials may be indistinguishable, or difficult to distinguish, in CT imaging data if the materials have similar attenuation coefficients at the energy of the CT scan. It has been found that such difficulty in distinguishing different materials may occur particularly at peak tube voltages that may be suitable for providing good image quality, for example 120 kV. Although materials may be more distinguishable if a lower peak tube voltage is used (for example, 80 kV), the image quality at such lower energies may be poorer, as the image may be more noisy.
A dual-energy, multi-energy or spectral CT system may acquire multiple, registered images at different energy levels. For example a dual-energy CT system may acquire a first image at a peak tube voltage of 120 kV and a second image at a peak tube voltage of 80 kV. The first image may be referred to as the high-energy image (or as an image obtained from a high-energy scan) and the second image may be referred to as the low-energy image (or as an image obtained from a low-energy scan).
A dual-energy CT system may acquire the high-energy image and low-energy image simultaneously, or substantially simultaneously, such that the voxels in the first image correspond to the voxels in the second image without requiring registration of the images. The images may then be considered as a single, combined set of image data comprising, for each voxel, an intensity value for the high-energy image (which may be referred to as a high-energy intensity value) and an intensity value for the low-energy image (which may be referred to as a low-energy intensity value). Each voxel also has an associated position in the coordinate space of the images (where the coordinate space for the high-energy image may be the same as the coordinate space of the low-energy image, for example as a result of simultaneous or near-simultaneous acquisition of the images).
Dual-energy (or multi-energy or spectral) CT may be used to separate materials by using both low-energy and high-energy image intensity values. Materials that exhibit similar attenuation at one of the scan energies may exhibit differing attenuation at the other of the scan energies. In some cases, materials with attenuations that are difficult to distinguish in the high-energy image may have attenuations that are easier to distinguish in the low-energy image. At the same time, using information from both the high-energy scan and the low-energy scan rather than just using information from the low-energy scan may overcome noise issues in the low-energy scan data.
The attenuation associated with some materials may be dependent on the material concentration or density. A more concentrated sample of the material may have higher attenuation (a higher CT value in Hounsfield units) than a less concentrated sample. The attenuation in the high-energy scan may change with concentration, and the (different) attenuation in the low-energy scan may also change with concentration.
A relationship may be derived between the change in attenuation with concentration in the high-energy scan and the change in attenuation with concentration in the low energy scan. It is known that, if low-energy intensity is plotted versus high-energy intensity, points representing different material concentrations may lie along, or near, a straight line on the plot of low-energy intensity versus high-energy intensity. It is also known that the slope of the straight line may be different for different materials, that is, that different materials may have a different relationship between change in attenuation with concentration in the high-energy scan and change in attenuation with concentration in the low-energy scan. Such differences may be due to properties of the materials, for example each material's atomic number. See, for example, Thorsten R. C. Johnson, Christian Fink, Stefan O. Schonberg, Maximilian F. Reiser, Dual Energy CT in Clinical Practice, Secaucus, N.J.: Springer, 2011.
It is well-known to use a contrast agent to increase the intensity of blood vessels as viewed in a CT image. Contrast-enhanced CT data (usually from a single-energy CT system) may be used for diagnosis or surgical planning relating to many medical conditions. For example, contrast-enhanced CT may be used for stenosis assessment, for example stenosis assessment of the coronary, renal or carotid arteries. Contrast-enhanced CT may be used to assess circuit perfusion, for example pulmonary circuit perfusion or circuit perfusion in the liver or in the brain.
In some circumstances, contrast-enhanced CT data (in which a contrast agent is used) and non-contrast-enhanced CT data (in which no contrast agent is used) are acquired for the same subject. Contrast-enhanced CT data and non-contrast-enhanced CT data may be used to create subtraction images in which, in principle, only the contrasted areas may be present (for example, subtraction images of blood vessels). The use of both contrast-enhanced CT data and non-contrast-enhanced CT data may require at least two CT scans to be taken, one with a contrast agent and one without a contrast agent.
Accurate identification of the contrast-enhanced blood pathway may be important in many uses of contrast-enhanced CT. However, accurate identification of the contrast-enhanced blood pathway may be challenging when calcium (for example, plaque or bone) is present. Calcium may appear with a similar attenuation to a contrast agent in the blood, for example an iodine-based contrast agent in the blood. It may be difficult to reliably distinguish calcium from contrast material.
FIG. 1 is a plot of mass attenuation coefficient (in cm2/g) against photon energy in keV. The CT attenuation of a material may be directly related to mass attenuation coefficient. In FIG. 1, the mass attenuation coefficient is plotted on a logarithmic scale. Mass attenuation coefficient against photon energy is plotted for three materials: iodine, calcium and water.
The change in CT attenuation of a material with different energies may be related to the Z number (atomic number) of the material. Iodine (Z=53) has its maximum attenuation at low energy and a lower attenuation at higher energies.
It may be seen that, on the plot of FIG. 1, the greatest difference between the attenuation of iodine and the attenuation of calcium may be seen at an energy of around 40 to 50 keV. A smaller difference is seen at 80 keV, and a still smaller at photon energies above 80 keV. Therefore it may be more difficult to distinguish calcium from an iodine-based contrast agent at higher energies than it is at lower energies.
The best scan energy for distinguishing calcium from iodine may be around 40 keV. However, using a scan at such a low energy may require higher currents than are preferred for scanner hardware, and lead to more noise in the image data. Therefore, a dual-energy CT scan at, for example, 80 kVp and 120 kVp may be used to aid distinction of iodine and calcium while maintaining acceptable current levels and noise performance.
A number of further issues may contribute to the difficulty of separating calcium from contrast agent. Issues such as noise, motion, contrast concentration, calcium density, object dimension, CT dose level, beam hardening and partial volume effects may cause each material to exhibit a different range of intensity values in different images, or in different parts of the same image. In this case, the materials are iodine and calcium, but similar effects may also apply to images of different materials.
Lower concentrations of contrast agents may be more difficult to distinguish from calcium than higher concentrations. However, lower concentrations of iodine may be required for certain patients, for example for patients with kidney issues.
Regions of calcium having different calcium density may produce different intensities, leading to a range of intensities for calcium.
Images taken with a lower CT dose may exhibit a greater spread of intensity values for a given material than images taken with a higher dose, and therefore make materials more difficult to distinguish. The separation performance may be strongly affected at low concentrations. However, images taken with a lower dose may be preferred in some circumstances as a lower CT dose means that the patient is exposed to less radiation.
Beam hardening is a change in the energy distribution of a CT beam as it passes through the body, such that it contains a higher proportion of higher (harder) energies. Beam hardening may be due to lower energies being absorbed first by the tissue. Beam hardening may result in different parts of an object having different intensities, even if the material is the same throughout the object.
The image may exhibit partial volume effects, in which voxels on a boundary of a first material and a second material have an intensity value that is a combination of that of the first material and that of the second material.
The above effects may lead to a given material exhibiting a different range of intensities in different images, or in different parts of the same image, adding to the difficulty of distinguishing one material from another. In some circumstances, the range of intensities of a first material (for example, calcium) in a given image may overlap with the range of intensities of a second material (for example, iodine) in that image.